38 IIOAV CROPS FEEI>. 



The result of tliese investigations is, tliat while, perhaps, 

 wilted foliage in a heavy rain may take u]) a small (juan- 

 tity of water, and while foliage and roots may absoib 

 some vajjor, yet in general and for the most part the at- 

 mospheric water is not directly taken up to any great ex- 

 tent by plants, and does not therefore contribute immedi- 

 ately to their nourishment. 



itmospheric VFater Enters Crops through the Soil.— 

 Jt is only after the water of the atmosphere has become in- 

 corporated with tlie soil, that it enters freely into agricul- 

 tiiial plants. The relations of this substance to proper 

 ve'jetable nutrition may then be most appropriately dis- 

 cussed in detail when we come to consider the soil. (See 

 p. 199.) 



It is probable that certain air-plants (epiphytes) native to the tropics, 

 ■which liave no connection with the soil, and are not rooted in a medium 

 capable of yielding water, condense vapor from the air iu considerable 

 quantity. So also it is pi'oved that the mosses and lichens absorb wafer 

 )an;ely from moist air, and it is well known that they become dry and 

 brittle in hot weather, recovering their freshness and flexibility when the 

 air is damp. 



§5. 



RELATIONS OF CARBONIC ACID GAS TO VEGETABLE 

 NUTRITION. 



Composition and Properties of Carbonic Acid, ~ 



When 12 grains of pure carbon are heated to redness 

 in 32 grains of pure oxygen gas, the two boiiies unite to- 

 gether, themselves completely disappearing, and 44 grains 

 of a gas are pj-oduced which lias the same bulk as the 

 oxygen had at the beginning of the experiment. The new 

 gas is nearly one-half heavier than oxygen, and differs in 

 most of its properties from both of its ingredients. It is 

 carbonic acid. This substance is the jiroduct of the burn- 

 ing of charcoal in oxygen gas, (H. C. G., p. 35, Exp. 6.) 

 It is, in fact, produced whenever any organic body is 



